The invention is generally related to the use of fiber optic technology for thermal radiation detection. More particularly, the invention provides a method and an apparatus for determining a temperature profile along a single optical fiber.
It is becoming widely recognized that various types of sensor devices can be advantageously made by using optical fibers either as the medium for data transmission, or as the sensor transducer, or both. Fiber optics for sensor systems may be useful in the presence of a high electromagnetic noise background or in environments where electrical signals cannot be used, such as in the presence of explosive atmospheres. Fiber optic sensor systems can be implemented in the measurement of a variety of parameters, such as current, pressure, moisture and temperature.
Temperature may be measured with optical fibers in several known ways. For example, optical fibers have been developed in which the light propagation characteristics of the fiber are dependent on temperature. In U.S. Pat. No. 4,151,747, an optical fiber cooperates with a light source and a detector for sensing changes in the temperature being monitored. The amount of light which passes through the fiber varies with changes in the temperature of the fiber. The disadvantage of this sensor is that the optical fiber is very difficult to manufacture because extremely accurate control of the optical and thermal properties of the fiber core and cladding is required. Another type of optical fiber temperature sensor measures the internally generated, black body radiation emitted from the fiber when it is heated. This sensor is taught in "Fiber-optic Temperature Sensor Based on Internally Generated Thermal Radiation", M. Gottlieb and G. B. Brandt, Applied Optics, Vol. 20, No. 19, October 1981. Both of the temperature sensors described therein only provide an indication of temperature along the hottest region of the fiber and thus function only as a "hot-spot" probe. Although such fiber optic "hot spot" probes are of potential value in a variety of applications, the existence of multiple hot spots along the fiber can, in some cases, significantly degrade the accuracy of the sensor.
It is an object of this invention to provide a method and an apparatus for determining a temperature profile along a single optical fiber by evaluating the spectral power density of the internally generated thermal radiation of an optical fiber.
Infrared radiation is emitted by all solid objects which are not perfectly transparent. This emission is often referred to as "black body radiation". According to the Stefan-Boltzmann law, the total rate per unit area of emission of energy of all wavelengths is directly proportional to the fourth power of the absolute temperature. And, according to Wien's law, the wavelength of maximum intensity is inversely proportional to the absolute temperature of the emitting body. All solid objects have an emissivity between zero and one; a perfectly transparent object which absorbs nothing and emits nothing, has an emissivity of zero, while an ideal, perfectly black object has an emissivity of one. Fiber optic material is a semi-transparent material with an emissivity value between zero and one.
The concept of monitoring temperature with an optical fiber by measuring the internally generated radiation of the fiber itself is disclosed in the article "Fiber-optic Temperature Sensor Based on Internally Generated Thermal Radiation" by M. Gottlieb and G. B. Brandt; Applied Optics; Vol. 20, No. 19; Oct. 1, 1981, the contents of which are incorporated herein by reference. In this article, a method of determining the hottest spot along a fiber is described. This method assumes that only one hot spot exists along the fiber and that this single hot spot generates nearly all the thermal radiation in the fiber. This radiation propagates through the fiber and is measured. A single number, the output voltage of the detector at the end of the fiber is converted into a temperature.
There are several assumptions made in the above-identified article. One hot spot exists along the fiber which has a constant temperature `T` across its length `l`. The hot spot is a distace `L` away from the detector. The length of the fiber, not including the `hot spot` is cold and contributes no thermal radiation. The distance `L` of cold fiber, attenuates the radiation emanating from the hot spot by a total amount of e.sup.-.alpha.L, where .alpha. is the absorptivity of the fiber and e is a constant (e=2.732), the base of the natural log. It is also assumed that the fiber is not perfectly clear and that some radiation passing through the fiber is lost because the fiber absorbs it. The clearness of the fiber, its absorptivity, is indicated by .alpha.. The lower the .alpha. the clearer or more perfect the fiber with .alpha.=0 being a perfect fiber. Accordingly, in a perfect fiber, if one watt of light enters the fiber (P.sub.in) and travels the length of the fiber, one watt will come out of the fiber (P.sub.out): P.sub.out =P in e.sup.-.alpha.L. Thus, if .alpha.=0: P.sub.out =P.sub.in e.sup.o ; or Pout=Pin. However, fibers are not perfectly clear and .alpha.&gt;0. It is also assumed by the Gottlieb et al. disclosure that the clearness, or the ability of the fiber to pass light, .alpha., is not dependent upon the color of light selected to measure fiber clearness. Accordingly, the absorptivity of the fiber is assumed to be a constant. Every part of the fiber is exactly the same as any other part, i.e., it has the same diameter, absorption constant, etc. It is also assumed that the fiber is in good, uniform, thermal contact with the object being monitored. Finally, the detector at the end of the fiber has a flat response and measures integrated power between two selected wavelengths.
The single hot spot sensor of Gottlieb et al. only provides an estimate of the hottest spot along a fiber's entire length. If in fact, two or more hot spots of similar temperature exist along the fiber, only one estimate of the hottest spot along the fiber is obtained, and in this situation, that value is obtained through a weighted sum of both hot spots. Accordingly, the weighted sum could yield a totally erroneous result.
In order to determine the temperature of an object at every point along the length of the optical fiber in contact therewidth, a temperature distribution monitor was developed and disclosed by the inventor of the instant invention is U.S. patent application Ser. No. 304,761, Distributed Fiber Optic Temperature Monitoring Apparatus and Method, filed Sept. 9, 1981, assigned to the assignee of the present invention and incorporated herein by reference.
U.S. patent application Ser. No. 304,761 discloses a multifiber bundle which monitors an entire temperature distribution. Each fiber of the bundle has a predetermined absorption constant .alpha. which is measurably distinct from that of every other fiber. This disclosure assumes that `N` hot spots exist along the fiber which has a temperature T.sub.1, T.sub.2 . . . , T.sub.N across its length and each hot spot temperature is a constant value across its length. It is further assumed that all parts of each fiber generate radiation which is transmitted to the detector as a weighted sum. Thus for a bundle containing N fibers, N values of integrated output power are obtained. Because each fiber in the bundle has a distinct absorptive constant .alpha., the attenuation and generating characteristics of each fiber are different, thus N values of integrated power are measured by N detectors. It is also assumed that absorbtivity .alpha. is constant with wavelength and that the total length and diameter of each fiber are homogeneous.
This multifiber bundle optical device provides an estimation of the actual thermal distribution, the accuracy of which is reflected in the number of fibers in the bundle. However, the device does utilize an integrated power output which is applied to a linear system of equations.